Z27AG first decoding pass: rpm, pedal, brake, handbrake, indicators, lights, doors, wheel speeds

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz
2026-10-13 20:47:20 +09:00
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 date: 2026-10-13
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 # Z27AG CAN: first decoding pass
 Session `z27ag-20261013-170050-checklist` (yukart-data `can-captures/`), checklist v1,
 117 marks. Method: list every transition of every rarely-changing bit and read it
 against the marks; for blinking signals, count toggles inside each marked range
 against the seconds before it; for values, dump bytes across the marked range; for
 speed, correlate every 16-bit word with GNSS speed over the crawls. Marks lag the
 action by a fraction of a second to a few seconds (the operator presses after doing),
 so a change shortly *before* a start mark is normal.
 Bit numbering: `ID.byte.bit`, byte 0 first, bit 7 the MSB.
 ## Found
 | Signal | Where | Evidence | Confidence |
 |---|---|---|---|
 | Engine speed | `210` bytes 4–5, BE, 1 rpm | idle 740–870; holds at ~1410 and ~1900 for "1500" / "2000" (operator: revs not exact); blip peak 2887 | High |
 | Engine speed, finer | `13D` bytes 0–1, LE, ≈ rpm × 2.048 | moves in lockstep with `210` | High |
 | Accelerator pedal | `210` byte 0, 0x00 released … 0xFA full (÷2.5 → %) | engine-off sweep: 0 → 0xFA → 0 | High |
 | Accelerator sensors 1 and 2 | `139` bytes 0 and 1 (raw, 0x33 rest … 0xEE full, track each other) | same sweep | High |
 | Throttle-related | `139` byte 2 (0x21 … 0xD5) | same sweep; plate or target | Medium |
 | Pedal-pressed flag | `210` byte 3 bit 7 | set exactly while the pedal is off rest | Medium |
 | Brake pedal switch | `231.4.1` (1 = pressed) | 3/3 presses, plus the foot-brake holds around the handbrake step and the crawls | High |
 | Handbrake | `239.0.2` (1 = applied) | set through every engaged stretch, clear during the crawls and the handbrake releases | High |
 | Left indicator | `423.2.6` blinking | 2/2 | High |
 | Right indicator | `423.2.5` blinking | 2/2 | High |
 | Hazards | `423.2.6` only blinks, 2/2 | odd: `2.5` stays still. Maybe `2.6` is "left or hazard", maybe the hazard has its own bit elsewhere at a rate this method missed | Low |
 | Side lights | `423.2.2` and `570.7.3` (1 = on) | on/off and during dipped | High |
 | Dipped beam | `423.3.1` (1 = on) | 1/1 | Medium (one sample) |
 | Any door open | `423.2.1` | driver and passenger | Medium |
 | Driver door | `423.0.4` | driver only, not passenger | Medium |
 | Driver belt unbuckled | `573.5.7` (1 = unbuckled) | set at ignition on, cleared at buckle; no release seen | Low |
 | A/C request / blower | `443` bytes 1–2 and `711` bytes 5–6, multi-bit | change with A/C and blower; not separated yet | Low |
 | A/C compressor engaged | `13D.6.7` | engine running, A/C on; set ~0.5 s after the request, cleared with A/C off | Medium |
 | Four wheel speeds | `235` four 16-bit words (bytes 0–1, 2–3, 4–5, 6–7) | zero parked, all four move together in the crawls | High (signal), **scale open** |
 | Vehicle speed | `231` byte 6 (and probably byte 7) | tracks the wheel words | Medium |
 **Wheel-speed scale.** Crawls topped out at ~6 km/h, and so far only the first byte of each word moved
 (0x4C at GNSS 3.5 km/h). Read as little-endian ×0.05 km/h (the forfour's `200`
 scale), that is 3.8 km/h, which is plausible given GNSS lag. Big-endian is not ruled out. The
 jack-stands session and a GNSS-matched drive settle both the byte order and the scale.
 ## Not on this bus (or not seen)
 - **Clutch pedal:** nothing changed in 3 presses, engine off.
 - **Gear position:** nothing in 5 gears, reverse or neutral. Expected for a manual. The
  gear can be computed from rpm ÷ wheel speed once the scale is known.
 - **Reverse:** no reverse bit found. The reverse lamp is probably wired directly.
 - **Steering angle:** not analysed yet (step 5 was done with the engine on, at the end).
 ## Not analysed yet
 - `312` (four noisy 16-bit words that follow the throttle: MAP/load/air candidates)
 - `449` temperatures (coolant candidate), `710`–`713` slow body/meter frames
 - `0C0`, `119`, `149`, `156` (counters/checksums; `149` inertial candidate)
 ## Scripts
 Scratch analysis for now: a session loader, a transition timeline, a blink counter and a
 GNSS correlator, in Python with numpy. They become a tool once a DBC exists to check
 against.